Discharge display devices {plasma display panels (PDPs)} using a scheme for performing a light emission by using gas discharging are roughly classified into an AC type discharge display device (AC type PDP) having one pair of discharge electrodes which are opposite to each other to cross through a discharge gas, each of which is constituted by a plurality of line-shaped electrodes, and both of the pair of discharge electrodes are covered by a dielectric layer, and a DC type discharge display device (DC type PDP) in which a pair of discharge electrodes both have metals on the electrode surfaces exposed to a discharge space. As an intermediate type discharge display device therebetween, a semi-AC type or semi-DC type display discharge device (semi-AC type or semi-DC type PDP) in which one discharge electrode of one pair of discharge electrodes is covered with a dielectric layer, and a metal on the electrode surface of the other discharge electrode is exposed to a discharge space is known.
There is also provided a color discharge display device (color PDP) in which infrared rays generated from gas discharging are irradiated on phosphor layers for emitting red, green, and blue lights to perform a color display. In this color discharge display device, the phosphor layer directly receives ion impact in a gas, or materials spattered by ion impact to the discharge electrode are accumulated on the surface of the phosphor, so that the phosphor must be prevented from being degraded.
Therefore, in a color discharge display device, first, the discharge electrodes must be strong against the ion impact. With respect to this point, an AC type discharge display device is advantageous. More specifically, in the AC type discharge display device, the discharge electrodes are covered with a dielectric layer such as low-melting-point glass or the like, and the surfaces of the discharge electrodes are covered with an electrode protecting layer which also serves as a secondary electron discharging material such as a magnesium oxide (MgO) or the like for protecting the electrodes from the ion impact. For this reason, it is not likely that materials spattered by the ion impact received by the discharge electrodes are accumulated on the phosphor layers, and high reliability can be obtained.
By the way, since in the AC type discharge display device, one pair of electrodes opposite to each other through a discharge space is not classified into anode and cathode electrodes, either of discharge electrodes may receive the ion impact. For this reason, an AC type discharge display device of an opposite-two-electrode type which has the simplest structure and can be easily manufactured is not easily used as a color discharge display device. Therefore, an AC type discharge display device of a surface-discharge three-electrode type in which a discharge electrode for a display is separated from an address electrode to assure an area on which a phosphor is coated has been practically used. However, the price of this AC type discharge display device is high because of a large number of electrodes. The high price hinders achievement of high resolution.
The problems of the opposite-two-electrode type discharge display device with respect to a conventional driving method will be described below with reference to FIG. 5 showing an example of the semi-AC type discharge display device serving as an opposite-two-electrode type discharge display device. The semi-AC type discharge display device shown in FIG. 5 is constituted by an AC type Y electrode 1 serving as one discharge electrode constituted by a plurality of line-shaped electrodes and a DC type X electrode 3 serving as the other discharge electrode constituted by a plurality of line-shaped electrodes, and the AC type Y electrode 1 and the DC type X electrode 3 are opposite to each other to cross through a discharge gas, i.e., are arranged in the form of a matrix.
The Y electrode 1 is constituted by line-shaped electrodes (transparent electrodes) covered with a dielectric layer 2, each having a predetermined width, and arranged at a predetermined interval, and is formed on a front-surface glass plate (not shown). The X electrode 3 is constituted by metal wires (stripe electrodes may also be used) each having a predetermined diameter, arranged at a predetermined interval, and consisting of stainless steel, nickel or the like each having a predetermined diameter and arranged at a predetermined interval, and the electrode surfaces of the electrodes are exposed to a gas space. The X electrode 3 is opposite to the inner walls of a large number of trenches 4 formed on a rear-surface glass plate 6 by an etching method, a sand blast method or the like to be close to or be in contact with the inner walls, and phosphor layers 5 for emitting red, green, and blue lights are formed to be sequentially and cyclically covered on the inner walls of the trenches 4.
FIGS. 1A to D show timing charts for explaining sustain discharging for memory discharging which is a prior art of a driving method for a discharge display device (the above mentioned semi-AC discharge display device in FIG. 5). The timing charts will be described below. Reference symbol Tad denotes an address period, and Tst denotes a sustain period.
FIG. 1C shows a waveform of a voltage Vxy between the X electrode 3 and the Y electrode 1. This waveform is an AC pulse waveform which is symmetrical with respect to positive and negative sides. In order to apply the voltage Vxy having the waveform shown in FIG. 1C across the X electrode 3 and the Y electrode 1, as shown in FIGS. 1A and B, two pulse voltages Vy and Vx which are negative pulses having the same waveform and have a predetermined phase difference are applied to the Y electrode 1 and the X electrode 3, or the voltage having the waveform shown in FIG. 1C may be applied to any one of the Y electrode 1 and the X electrode 3, and the voltage of the other electrode may be set to be zero.
FIG. 1D shows a discharge keeping pulse applied to one pair of display electrodes, i.e., the Y electrode 1 and the X electrode 3 and only a change in electrode surface potential caused by wall charges generated by the discharge keeping pulse. A description of the process in which wall charges depending on a picture screen are formed on a selected cell by an address operation performed prior to the change in electrode surface potential will be omitted. More specifically, the explanation is made on the sustain period Tst where the wall charges have been formed on the Y electrode 1 and the X electrode 3 or both the electrodes in an address period Tad, and the memory discharging is performed by applying the discharge keeping pulse.
A state that negative wall charges are formed in the address period Tad on the Y electrode 1 serving as an AC type electrode is assumed, and the pulse voltage Vy having a waveform shown in FIG. 1A is applied to the Y electrode 1 in the sustain period Tst. Since the other electrode X3 is a DC type electrode, no wall charges are formed on the X electrode 3. However, a pulse voltage Vx shown in FIG. 1B and having a phase difference of 180.degree. with respect to the pulse voltage shown in FIG. 1A is applied to the X electrode 3.
In this manner, since positive and negative charges generated by wall charges when respective pulse voltages are applied are alternately reversed and superposed one another, the voltage Vxy applied across the X electrode 3 and the Y electrode 1 becomes the an AC pulse voltage having a waveform shown in FIG. 1C. More specifically, as shown in FIG. 1D, since it is assumed that negative charges are accumulated on the Y electrode 1 first, the voltage superposed with the voltage Vy having the waveform shown in FIG. 1A exceeds a discharge start voltage Vb1. For this reason, a first discharging occurs. Then, the negative charges on the Y electrode 1 are eliminated, and, subsequently, positive wall charges are formed. Since the wall charges boost the electrode surface potential of the Y electrode 1, when a negative pulse is applied to the X electrode 3 as shown in the waveform of FIG. 1B, a second discharging occurs, and negative wall charges are generated on the Y electrode 1 again. In this manner, continued keeping discharging is performed. Since no charged particles have been left in a discharge space at the start of the second discharging, the conditions at the start of the second discharging are almost the same as those at the start of the first discharging. For this reason, a second discharge start voltage Vb2 is a high voltage equal to the first discharge start voltage Vb1.
According to the driving method of the prior art described with reference to the timing charts in FIG. 1, by the sustain waveform applied, both the electrodes are symmetrically positive and negative, so that either of the electrodes is on the negative side at the same probability. At this time, the electrode necessarily receives the ion impact. Therefore, a place on which a phosphor layer is coated must be set at a position except for a position on the electrodes and near the electrodes. However, in a discharge display device having a fine discharge space, the place cannot be easily assured.
In addition, by the sustain waveform of the first prior art, generation of wall charges is ended by each discharging upon pulse application, and no charged particles have existed in the discharge space, and the next pulse is applied at a timing at which the number of metastable atoms becomes small. For this reason, since discharging always occurs in a state wherein a priming effect is small, a start voltage is high, and the ion impact increases because of the high start voltage.
In consideration of the circumstances, according to the present invention, in a driving method for an AC type discharge display device having a simple structure and a two-electrode structure which can be easily manufactured, there is provided a driving method which can decrease influence of ion impact on a discharge electrode or a phosphor and at the same time can cause the discharge display device to have the same memory function as that of a conventional AC type discharge display device.